If a video-receiver is to be fully integrated, then a monolithic filter must be used, as opposed to a partially integrated or a discrete receiver using a discrete filter. In the case of a partially integrated receiver using a discrete filter, a problem is created in that the system would have to be provided with a board, and more than likely, a metal case around it, which is relatively costly.
Fully-monolithic receivers differ from their discrete counterparts primarily due to a lack of low-loss monolithic inductors. For this reason, most of the filtering function on such receivers must be realized with active simulations of inductors. However, active fully-monolithic intermediate frequency (IF) filter-amplifiers suffer from detrimental noise characteristics of the active devices required to simulate inductors. This performance is further aggravated in the case of an IF filter-amplifier providing additional high-gain to the channel.
In a discrete realization, placing a variable gain amplifier (VGA) after an IF filter, helps its noise performance by band limiting, since a discrete inductance/capacitance (LC) IF filter is practically noiseless. However, in a fully-monolithic circuit, placing a high-gain VGA after the IF filter, which is the major source of noise, results in a substantially increased noise level leading to a reduced signal-to-noise (S/N) ratio. Moreover, in such a structure, the distortion of the filter changes with a widely varying signal, such that if the linearity is made sufficient for the highest signal levels, it becomes suboptimal for the low signal levels.
In the case of low-VGA gain, the IF channel noise performance is acceptable. However, in the high-gain case, the amplitude of the input signal to the filter is the amplitude of the output signal divided by the gain of the VGA, and such a reduced input signal to the filter results in a substantial deterioration of the system S/N ratio. The S/N ratio is defined as the ratio of the maximum undistorted rms input signal of the filter to the input referred noise of the filter.
Accordingly, there is needed a fully monolithic IF channel which allows a wide gain range while still maintaining an acceptable S/N ratio, all without using off-circuit LC devices.
It is, therefore, an object of the present invention to provide a fully-monolithic IF channel receiver system that has the improved signal processing noise capabilities.
Another object of the invention is to provide a fully-monolithic receiver, therefore reducing its manufacturing cost.
Yet another object of the present invention is to provide the IF band-pass filter with the maximum possible input signal in order to maintain the signal/noise ratio.
A further object of the present invention is to provide a constant signal level input to the IF band-pass filter, and therefore, optimize its linear range and distortion, and save dissipated power.